The present invention relates to novel liquid urethane polyisocyanate compositions based on polyisocyanates or polyisocyanate mixtures of the diphenylmethane series and on certain polyether polyols of relatively high functionality. The present invention also relates to a process for the preparation of such polyisocyanate compositions. The invention further relates to the use of such compositions as the polyisocyanate component in the production of flexible polyurethane foams, particularly molded flexible polyurethane foams.
It is known that 4,4'-diisocyanatodiphenylmethane or polyisocyanate mixtures of the diphenylmethane series containing 4,4'-diisocyanatodiphenylmethane as their principal component can be modified with substoichiometric quantities of polyether polyols in such a way that the resultant isocyanate semiprepolymers show a reduced tendency for solid 4,4'-diisocyanatodiphenylmethane to separate. Such modified materials are liquids that are stable at room temperature and even lower temperatures. See, for example, DE-PS 1,618,380, GB 1,369,334, DE-OS 2,913,126, DE-OS 2,404,166, DE-OS 2,346,996, DE-AS 2,737,338, DE-AS 2,624,526, DE-OS 2,513,793, DE-OS 2,513,796, EP-A 010,850, EP-A 022,617 or EP-A 111,121. Among the polyisocyanate compositions that are increasingly being used for the production of flexible polyurethane foams, especially cold-hardening molded flexible polyurethane foams, are certain isocyanate semiprepolymers, by which are meant mixtures of excess starting polyisocyanate with urethane-modified starting polyisocyanate. In particular, such isocyanate semiprepolymers include those based on polyisocyanates or polyisocyanate mixtures of the diphenylmethane series and on relatively high molecular weight polyether polyols of the type described, for example, in EP-A 010,850 or EP-A 111,121. Applications for foams of this type include, for example, upholstery materials used in vehicles or for furniture and carpet backings.
It has now been found that isocyanate semiprepolymers based on polyisocyanates or polyisocyanate mixtures of the diphenylmethane series and on polyether components consisting of at least one polyether polyol having a hydroxyl equivalent weight of about 500 to about 3000 and a hydroxyl functionality of greater than about 4 are room-temperature liquids that are valuable starting materials for the production of flexible polyurethane foams. The polyisocyanate mixtures of the present invention, which are described in more detail below, have the following advantages:
1. It is possible to produce flexible foams having a comparatively low gross density but having the same favorable mechanical properties.
2. The isocyanate index range is broad and, hence, the potential hardness range for the same polyol component is also broad. In other words, typical flexible molded foam formulations may readily be processed within the isocyanate index range of 80 to 120 using the polyisocyanate mixtures of the invention.
3. The polyisocyanate mixtures of the invention allow the preparation of flexible foams using large quantities of water as blowing agent. That is, up to 6% by weight based on the polyol component can be used. Such quantities are atypical of MDI-based flexible foams. As a result, molded foams having gross densities below 40 kg/m.sup.3 may be obtained without using physical blowing agents.
4. The mechanical properties of the foams, particularly their tensile strength and elongation at break, are excellent. This is surprising because the use of relatively high molecular weight polyether polyols having a hydroxyl functionality above 3 (particularly above 4) as part of the polyol component that is reacted with the polyisocyanate component is completely atypical for the preparation of flexible polyurethane foams. In general, the use of such polyols actually leads to a reduction in tensile strength.